1. Field of the Invention
The field of this invention is internal combustion engines and, more specifically, such engines utilizing stratified air-fuel mixtures at the engine intake manifold and intake valve. Both spark ignition and compression ignition internal combustion engines are included.
2. Description of the Prior Art
Many spark ignition engines of the prior art have utilized stratified air-fuel mixtures at the engine intake manifold and/or intake valve (see for example references A, B, C, D, E, F). In these prior art, spark ignition, stratified mixture, engines, two kinds of air-fuel mixtures are utilized differing in the ratio of air to fuel, and any one kind of air-fuel mixture is contained within a single continuous region and is not broken up into several small regions separated from one another by regions of another kind. As a result whenever any portion of such a single continuous region is ignited by a spark, or any other ignition means, the entire region burns fully within a short time interval thereafter. This latter circumstance means compression ignition is impractical to use as a means of igniting these continuous regions, since compression ignition completes the burning of the entire region extremely quickly and strong pressure waves and an extremely loud engine noise are the consequence. Hence these prior art stratified mixture spark ignition engines can only use a spark or a flame from a spark for the igniting of the air-fuel mixture within the engine cylinder. As a result only those air-fuel mixtures which are spark ignitable or flame ignitable are used in these prior art stratified mixture engines. This type of engine intake stratification is hereinafter termed "two barrel carburetor stratification" since this is the most common manner of securing this type of stratification. In some cases of the prior art, in addition to one or two continuous air-fuel mixture zones as described above, an additional, continuous, air-only zone is also included in the stratified intake mixture. The term "two barrel carburetor stratification" is intended hereinafter to also include this type of intake stratification.
Experiments in engines show that air-fuel mixtures leaner in fuel than about 24 lbs. of air per lb. of fuel cannot be spark ignited (see for example reference 6) and that air-fuel mixtures leaner in fuel than about 27 lbs. of air per lb. of fuel cannot be flame ignited (see for example reference H). These then are the leanest mixtures useable in prior art, stratified mixture, spark ignition, engines.
The use of very lean mixtures is desired in order to reduce the undesireable exhaust emissions of oxides of nitrogen. Experiments in engines show these emissions to decrease as air-fuel mixtures leaner in fuel are supplied to the engine (see for example references I and J). For prior art, stratified mixture, spark ignition engines, this beneficial effect of leaner mixtures to reduce oxides of nitrogen can only be utilized up to the flame or spark ignition limit of mixture ratio described above.
Extremely lean mixtures, at least as lean as 45 lbs. of air per lb. of fuel, can readily be compression ignited (see for example references K and L) and in this way greater reductions of exhaust emissions of oxides of nitrogen can be achieved than is possible with prior art, stratified mixture, spark ignition engines. When, however, compression ignition is used with air-fuel mixtures premixed according to the prior art in the engine intake manifold, strong pressure waves and in consequence unacceptably loud engine noise are produced (see for example reference M).
The diesel engine has long used compression ignition, and of very lean overall air-fuel ratio, without creating excess engine noise. Admittedly a diesel engine is commonly somewhat noisier than a spark ignition engine but is nowhere near as noisy as a prior art spark ignition engine in which a large continuous portion of the mixture is being compression ignited. The reason why the diesel engine can use compression ignition without excess noise must be sought in the manner of creating the air-fuel mixture which is then compression ignited.
In the diesel engine the air-fuel mixture is not created in the intake manifold but rather by injecting the liquid fuel into the already compressed air in the cylinder only a short time before ignition. As a result the air-fuel mixture which is compression ignited is stratified, in a particular way, since there is not enough time between injection and compression ignition for this liquid fuel to first evaporate and then diffuse fully into the surrounding air mass. The particular kind of stratification obtained in a diesel engine is hereinafter referred to as "injected liquid spray" type of stratification of an air-fuel mixture. This injected liquid spray kind of stratification is obtained when a liquid fuel is injected under high pressure into an air mass and atomized into many separate liquid drops. Fuel evaporates from each drop and diffuses gradually into the surrounding air and thus creates a zone of air-fuel mixture around each drop. Within each such zone the full range of air-fuel ratios exists varying continuously all the way from, very fuel rich next to the liquid surface of the drop, to very fuel lean fartherest away from the drop surface. The total number of fuel containing zones created is essentially equal to the number of liquid drops created by atomization. These many fuel containing zones are separate, discontinuous and essentially alike in having a wide range of air-fuel mixture ratios. In addition, one or more air-only zones may be created in those parts of the air mass into which no liquid fuel was injected. These evaporation and diffusion processes which create the injected liquid spray type of stratification are described in references N, O and P.
An air-fuel vapor mixture can be ignited by compressing it adequately within a piston and cylinder chamber. Ignition does not occur immediately upon compression but only after a compression ignition time delay interval whose length varies with the degree of compression, the air-fuel ratio of the mixture, and the type of fuel in the mixture (see for example reference Q). In contrast to spark ignition, compression can be used to ignite air-fuel vapor mixtures of almost any air-fuel ratio provided only that adequate compression is applied. Hence extremely lean air-fuel mixtures can be compression ignited which could not be spark ignited. The burning process which occurs after an air-fuel mixture is compression ignited occurs with extreme rapidity as shown in reference R. The mechanism of this extremely rapid burning is a subject of controversy at this time but, if it does take place via the travel of a burning zone through the air-fuel mixture, then there is general agreement that the velocity of travel of this burning zone must be of the order of several thousands of feet per second, a velocity roughly ten times faster than that of the normal, spark ignited flame. This extremely rapid burning and energy release from compression ignition creates strong gas pressure waves in the cylinder (see for example reference S) and, in consequence, a very loud engine noise.
The diesel engine uses only compression ignition of an air-fuel mixture created by the injected liquid spray technique as described heretofore. The strong pressure waves, characteristic of compression ignition, are generated separately in each ignition region around each fuel droplet and, not being coordinated between regions, these several individual pressure waves occur at different times and travel in different directions and do not act together to increase engine noise. A consequence of this time and position dispersed occurence of compression ignition in the diesel engine is that this engine is much quieter running than is a compression ignition engine utilizing a homogeneous, premixed air-fuel mixture. These latter engines are so noisy as to be unsuitable for any ordinary engine application. Hence we see that the combustion noise of compression ignition can be reduced to acceptable levels by having the compression ignition processes occur separately in individually small volume regions and at different times between regions.
In a diesel engine, after compression ignition has occurred, the unevaporated liquid fuel portions and the over-rich-in-fuel portions, present in each region of the stratified air-fuel mixture, burn only with difficulty and some of these portions become soot which survives to exhaust in the form of exhaust smoke, an undesirable exhaust emission of the diesel engine. The effects of fuel evaporating ability upon exhaust smoke emmissions of diesel engines is described in reference AA.
The most common type of spark ignition engine in use today, the gasoline engine, uses spark ignition in combination with a single barrel or two barrel carburetor for creating the air-fuel mixture. The resulting air-fuel mixture must be within the spark ignitability limits and in consequence engine torque is controlled by throttling the engine intake mixture. The result is a loss of engine efficiency due to the necessity of pumping out the exhaust gasses at full atmospheric pressure. The magnitude and deleterious effects of this pumping loss are described in reference U. The normal flame, started by spark ignition, cannot reach all the way to the chilled surfaces of the combustion chamber and the thin film of air-fuel mixture next to the surfaces of the combustion chamber fails to burn and emerges as unburned hydrocarbon emmissions in the engine exhaust. Although these undesirable emmissions can be reduced by increasing the air-fuel ratio, only limited improvements are available within the spark ignition inflammibility limits. These surface film effects on hydrocarbon emmissions are described in reference T and the effects thereon of air-fuel ratio are discussed in reference V. When the compression ratio of a gasoline engine is increased in order to increase the efficiency of the engine compression ignition of the last burning portions of the homogeneous air-fuel mixture may occur and the consequent locally rapid rate of pressure rise causes the undesirable noise known as knock. Although knock can be prevented by increasing the octane rating of the fuel being used such higher octane fuels are more difficult to prepare and thus are more costly to use. The compression ignition process of gasoline engine knock occurs in a single moderate sized volume of essentially uniform and homogeneous air-fuel mixture and hence the pressure wave, characteristic of compression ignition, is a single, strong pressure wave which greatly increases engine noise.
In a spark ignition engine, whose air-fuel mixture is stratified at the time of combustion by containing some air only regions or by containing some regions too lean for spark ignition, special ignition arrangements are sometimes needed to assure that spark ignition of the spark ignitable air-fuel mixture regions will take place at the proper time in the engine cycle. Various kinds of arrangements have been used in the prior art for this purpose including:
(1.) Locating the spark plug in the combustion chamber at a place where a spark ignitable region of the stratified air-fuel mixture is also located. PA1 (2.) Using a long duration spark discharge when the stratified air-fuel mixture is moving. PA1 (3.) Using two or more spark plugs (multiple spark plugs) located at different places in the combustion chamber. PA1 (1.) Engine exhaust emissions can be reduced by use of leaner air-fuel mixtures. PA1 (2.) With spark ignition, air-fuel mixtures leaner than about 24 to 1 or at most 27 to 1 cannot be used as these are not spark ignitable. PA1 (3.) With compression ignition air-fuel mixtures at least as lean as 45 to 1 and probably leaner can be used. PA1 (4.) Compression ignition of a uniform air-fuel mixture produces excessive engine noise. PA1 (5.) The noise of compression ignition can be reduced to acceptable levels by so stratifying the air-fuel mixture that small volumes of air-fuel mixture are compression ignited at different times, producing a time and position dispersed occurence of compression ignition. PA1 (6.) Control of spark ignition engine power output by throttling the flow of the intake air-fuel mixture increases the engine friction power loss and hence reduces efficiency. PA1 (7.) The spark ignitable portions of a stratified air-fuel mixture can be spark ignited by proper spark plug location, by use of multiple spark plugs, by use of long duration spark discharge or by a combination of these methods. PA1 (8.) If the engine air-fuel mixture is stratified in the intake manifold stratification is retained to the time of combustion. PA1 1. Supply a separate fuel to each of several different channels, these several fuels differing in the kinds and proportions of hydrocarbons or other fuel components present. PA1 2. Supply a separate fuel to each of several different channels, these several fuels differing in the amount or type of antiknock compound present. PA1 4. Adjust the air-fuel mixing devices in each of several different channels so that they furnish different proportions of air to fuel in the air-fuel mixtures supplied to the stratifier valve by these separate channels.
To assure spark ignition of the spark ignitable regions requires only that a spark be present in the plug gap with the spark ignitable region also at the plug. In this way spark ignition of the spark ignitable regions can be secured by using one or a combination of the foregoing arrangements as is well known in the art.
That an air-fuel mixture, stratified in the engine intake manifold, will retain this stratification throughout the intake and compression processes, and thus be stratified at the time of combustion, can be shown in several ways of which the following are typical examples:
(a.) A four stroke cycle, spark ignition, single cylinder gasoline engine was supplied with gasoline from an injector valve, of diesel engine type, positioned in the intake manifold so as to spray liquid gasoline into the incoming intake air stream. Fuel was supplied under an atomizing pressure of 1000 psi to the injector valve by an injection pump, also of diesel engine type, actuated by the engine camshaft. The characteristic of this injection pump, like most diesel injection pumps, is to deliver the entire fuel quantity in about 10 to 20 degrees of crankshaft rotation (this being desirable for correct running of a diesel engine). The situation existing in the intake manifold of this engine is as follows: as the piston descends on the intake stroke, the air flows continuously through the intake manifold and into the cylinder during the entire 180 degrees of crankshaft rotation of the intake stroke, but the fuel is sprayed into this intake air mass in a brief spurt of only 10 to 20 crankshaft degrees, and in this way a stratified intake fuel-air mixture is created in the intake manifold. That air mass into which the fuel was sprayed becomes a fuel rich air-fuel mixture, whereas the other air portions are lean in fuel or free of fuel. The fuels tested were primary reference fuels (i.e., mixtures of normal heptane and iso-octane) whose boiling points lie between 209.degree. F. and 210.degree. F., and the intake air to the engine was heated to 300.degree. F. Hence all fuel was evaporated and no liquid portions remained. The intake valve of the engine was fitted with a 180 degree shroud, oriented and fixed so as to direct the incoming air-fuel charge in a tangential flow direction within the engine cylinder. This arrangement of a shrouded intake valve to achieve tangential rotary motion of the air-fuel mixture within the engine cylinder is shown in FIGS. 2 and 3 of reference W. With these engine arrangements, a stratified intake mixture was created in the intake manifold, this mixture was then drawn into the cylinder in a manner to set the mixture into rotation within the cylinder; the stratified and rotating mixture was then compressed by the piston into the pancake-shaped combustion chamber.
If the mixture stratification, created in the intake manifold, failed to survive the intake and compression processes then the combustion in this test engine would not be changed in any way when the time of fuel injection into the intake manifold was changed to different portions of the intake stroke. But in these experiments the very opposite was found; the power, efficiency and knocking tendency of this engine were greatly changed when only the time of fuel injection into the intake manifold was changed to different portions of the intake stroke. At certain fuel injection timings the engine misfired since the rotating, and still stratified, air-fuel mixture in the combustion chamber had placed a mixture ratio, too lean for spark ignition, in front of the spark plug at the time of spark firing. The observed changes in power and efficiency with fuel injection timing change clearly demonstrate that the rotating air-fuel mixture in the engine combustion chamber had retained the stratification created in the intake manifold, which had thus survived the intake and compression processes.
(b.) Another engine experiment, similar in several ways to the foregoing example, is reported in reference X by Mr. J. Munot. The engine arrangements, fuel injection method, and intake stratifying technique of Mr. Munot are essentially the same as in the foregoing example except that the engine intake valve was not shrouded and the time of fuel injection into the engine intake manifold was not varied but remained fixed at 70 crankshaft degrees after piston top dead center during the intake stroke. Mr. Munot varied the overall air-fuel ratio, measured the engine power output, and observed that maximum engine power output occured at overall air to fuel weight ratios between about 15 to 1 and 21 to 1. As is well known in the art, maximum power air to fuel weight ratio lies between about 11 to 1 and 13 to 1 when the air-fuel mixture is uniform and not stratified. The very full lean air-fuel ratios for maximum power observed by Mr. Munot border in some tests on the spark ignitable range and can only be explained to be the result of a stratification of the air-fuel mixture, created in the intake manifold and retained through intake and compression processes up to the time of spark ignition and combustion.
(c.) In reference Y, Gau describes engine experimental results which demonstrate the general principal that stratification readily survives the pulsating flow conditions characteristic of internal combustion engines. Gan shows that stratification created by the combustion process survives the expansion and exhaust processes and is retained in the exhaust gases. In an engine exhaust system gas velocities and turbulence are much higher than in an engine intake system and shock waves and supersonic flow are additionally present. Hence the opportunities for mixing and elimination of stratification are much greater in the exhaust process than they are in the intake process. Thus if stratification can survive the exhaust process, as shown by Gau, it can readily survive the intake process in an internal combustion engine.
(d.) In reference Z, Jessel et al describe engine experiments which show that stratification existing at intake survives to combustion.
In summary, the foregoing description of that portion of the internal combustion engine prior art, relevant to this invention, shows the following: